1. Field of the Invention
The present invention relates to a method of low-temperature and near neutral-pH precooking for the production of corn flour and, more particularly, to one that achieves continuous partial hydrolysis of the corn insoluble fiber and avoids excessive pregelatinization with a xylanase as a processing aid during the manufacture of an instant corn flour for the elaboration of arepa and tortilla and derivatives.
2. Description of Related Art
The production of high-quality masa flour can be produced by conventional techniques only if the food-grade dent corn has the following characteristics: uniformity in kernel size and hardness, low number of stress-cracks and kernel damage and ease of pericarp removal during the lime-water cooking process. The mature kernel has four separable components, on a dry weight basis: tip cap (0.8-1.1%), pericarp or bran (5.1-5.7%), endosperm (81.1-83.5%), and germ (10.2-11.9%). In dry or wet-milling processes the bran includes the pericarp, tip cap, aleurone layer and adhering pieces of starchy endosperm as well. Nixtamalized corn flour (NCF) is produced by the steps of alkaline cooking of corn, washing, milling the nixtamal and drying to give corn masa flour. This flour is sieved and blended for different product applications and it is usually supplemented with additives before packaging for commercial table or packaged-tortilla and snack production. Although the pericarp or bran is partially removed during the alkaline-cooking and washing process stages, there is still fiber left from the corn kernel (Montemayor and Rubio, 1983, U.S. Pat. No. 4,513,018). Whole Nixtamalized corn flour or masa flour can contain from 7-9% of total dietary fiber or bran with 6-8% mainly consisting of insoluble fiber on a dry basis (Sustain, 1997).
The cell walls or non-starch polysaccharides (NSP) are the major corn dietary fiber components and are composed of hemicellulose (heteroxylan or pentosan and xcex2-glucan: 4.4-6.2%), cellulose (2.5-3.3%) and some lignin (0.2%). According to Watson (1987: Tables IV and VII), the corn pericarp makes up 5-6% of the kernel dry weight. This pericarp also contains 90% insoluble fiber (67% hemicellulose and 23% cellulose) and only 0.6% soluble-fiber (soluble-arabinoxylan and xcex2-glucan). It is estimated that dietary fiber in both pericarp or bran (4.9%) and endosperm (2.6%) make up 80% of the total dietary fiber. The corn insoluble fiber is mainly found in the pericarp and endosperm (aleurone and starchy) which make up 68% of the total dietary fiber (9.5% in a dry-weight basis).
Unlike corn endosperm, in which soluble fiber amounts to 12% of the total fiber (4.1%), in whole wheat, soluble fiber represents 22% of total fiber (about 20% of the flour water-uptake is bound to the soluble pentosan fraction). Arabinoxylan is a complex polymer (20,000-170,000 Daltons) with a linear backbone of (1,4)-xcex2-xylopiranosyl units to which substituents are attached through O2 and O3 atoms of the xylosil residues (mainly, xcex1-L-arabinofuranosyl). This polymer is apparently linked to the cellulose skeleton in the corn cell wall by ester linkage cross-bonding through ferulic and diferulic acid (Watson, 1987). However, heteroxylan insolubility in corn bran might be due to protein-polysaccharide linkages and a highly branched structure (23% of the xylan backbone does not bear side-chains) as opposed to wheat bran (Saulnier et al., 1995).
During alkali-cooking and/or steeping, there are chemical and physical changes such as nutrient losses along with partial pericarp or bran removal, degradation of the endosperm periphery with starch gelatinization/swelling and protein denaturation in the precooked corn kernel. The most important nutritional modifications are: an increase in the calcium level with improvement in the Ca to P ratio, a decrease in insoluble dietary fiber and zein-protein, a reduction in thiamin and riboflavin, an improvement of the leucine to isoleucine ratio reducing the requirement for niacin, and leaching the aflatoxins into the wastewater (Sustain, 1997).
The known cooking methods (batch or continuous) have been proposed as the critical variables (Sahai et al., 2001) which determine soluble-solid loss (1% to 1.6% COD) in limewater residue for anaerobic biodegradation (Alvarez and Ramirez, 1995). Dry solid matter (1.5%-2.5%) includes an average of 50-60% dietary fiber, 15-20% ash, 15% starch, 5-10% protein and 5% fat. Bryant et al., (1997) showed an optimum change in starch behavior at a lime level similar to the corn masa industry where starch gelatinization indicators (enzyme digestion, water retention capacity, starch solubility and DSC-peak temperature=69xc2x0 to 75xc2x0 C.) are increased with lime addition of 0 to 0.4%, peaking at 0.2%. They also found a peak-viscosity temperature reduction upon the addition of lime up to 0.5%, indicating faster granule swelling that requires less thermal energy. Corn pericarp nixtamalization (Martinez et al., 2001) has a first-order stage associated with a fast dissolution of hot-water soluble fractions as starch and pectin, and alkali-soluble fat. A second stage is due to a slow alkaline-hydrolysis of the hemicellulose-cellulose-lignin structure with a higher hemicellulose loss proportional to alkaline-pH concentrations.
Arabinoxylan degrading enzymes include xylanases (1,4-xcex2-D-xylan xylanohydrolase, EC 3.2.1.8) and xcex2-xylosidases (1,4-xcex2-D-xylan xylohydrolase, EC 3.2.1.37). The former endozyme randomly hydrolyze the insoluble and soluble xylan backbone (EC 3.2.1.8) whereas the latter exozyme hydrolyze xylose from the non-reducing end of the xylose-polymer (EC 3.2.1.37). Xylose is not usually the major product and it is typically produced after xylobiose and xylotriose (smallest oligomer). Virtually all xylanases are endo-acting as determined by chromatography or their kinetic properties (substrate and product formation), molecular weight and pH (basic or acidic) or its DNA sequence (crystal structure). They can be structurally classified into two major families or isoenzymes (F or 10 and G or 11) of glycosyl hydrolases (Jeffries, 1996). F11 xylanases are larger, with some cellulase activity and produce low DP oligosaccharides (less specific); F11 are more specific for xylan and with lower molecular weight (i.e., B. Circulans and T. harzianum).
In addition, the Enzyme Technical Association (ETA, 1999; FDA, 1998) classified as carbohydrases the following hemicellulases (trivial name): a) endoenzymes (EC 3.2.1.32=1,3-xcex2-xylanohydrolase, 78=mannanohydrolase and 99=arabinohydrolase) and b) exoenzymes only attack branches on the xylose-polymer (pentosan), producing xylo-oligomers (EC 3.2.1.55=xcex1-L-arabinofuranosidase, glucuronic-acid glycosilase and ferulic-acid esterase).
Currently recognized endoenzymes (xylanases) and exoenzymes produced from A. niger (EC 3.2.1.8 and 37,55), A. oryzae (EC 3.2.1.8 or 32), B.subtilis (EC 3.2.1.99), and Trichoderma longibrachiatum (formerly reseei: EC 3.2.1.8) are Generally Recognized As Safe (GRAS; 21 CFR 182, 184 and 186) and require no further approval from the U.S. Food and Drug Administration or Recognized As Safe (RAS in Europe: Mathewson, 1998). However, direct and indirect food additives (i.e., packaging materials) are regulated in 21 CFR 172 and 174-178 as well. Secondary direct additives, a sub-class of direct additives, are primarily Processing Aids which are used to accomplish a technical effect during food processing but are not intended to serve a technical or as a functional additive in the finished food. They are also regulated in 21 CFR 173 (Partial List of Enzyme Preparations that are used in foods). Finally, all GRAS Substances produced through recombinant-DNA which were widely consumed prior to 1958, and which have been modified and commercially introduced after 1958 must comply with regulatory requirements proposed in 21 CFR 170.3 (GRAS Notice).
The benefits of using a commercial xylanase (endoenzyme) in cereal flours instead of a non-specific hemicellulase (endo/exoenzyme) preparation are a reduction in side activities (cellulase, beta-glucanase, protease and amylase) and a reduction of dough-stickininess. Arabinoxylan degrading enzymes with well defined endo-acting and exo-acting activities have become commercially available, for food and feed, from the following companies: Amano, Danisco-Cultor, EDC/EB, Genencor, Gist-Brocades, Iogen, Novo, Primalco, Rhodia and Rohm.
Suggested applications for commercial xylanases (endopentosanases) and hemicellulases (pentosanases) mentioned in the literature include: 1) improving the watering of spent grains and energy reduction during grain drying; 2) facilitation of dough formulation with less water, reduction of stickiness in noodle and pasta production; 3) reduction in the water content when formulating grains for flaking, puffing or extrusion; 4) retarding staling or hardening in bread; 5) relaxing dough for cookie and cracker production and use of sticky cereal flours in new product formulations; 6) increase in bran removal when added to tempering water; and 7) reducing both steeping time and starchy fiber in corn wet milling.
A moderate exoxylanase addition decreases water uptake in wheat dough, whereas using an endoxylanase increases water binding and soluble-xylans as well for a high-moisture bread product. On the contrary, if starch gelatinization is to be minimized, a higher endozyme addition is desirable and hydrolysis of the soluble fraction releases water for low-moisture cookie or cracker products (EPA Patent0/338787). Therefore, a suitable level of xylanase results in desirable dough softening without causing stickiness, thereby improving machinability during forming and baking operations.
Haarasilta et al. (U.S. Pat. No. 4,990,343) and Tanaka et al. (U.S. Pat. No. 5,698,245) have proposed that the use of hemicellulase and pentosanase (Cultor and Amano) causes decomposition of wheat insoluble fiber for bread volume increase. Van Der Wouw et al. (U.S. Pat. No. 6,066,356) also reported the use of recombinant-DNA endo-arabinoxylanase (Gist Brocades) breaks down the water-insoluble-solids (xcx9c1.5%) from degermed maize and increases their in-vitro digestion (13%-19%) for animal feed or in wheat flour for improving bread volume (xcx9c9%).
A pilot process (WO Patent 00/45647) for the preparation of a modified masa foodstuff used a reducing agent (metabisulfite) or an enzyme as a processing aid (disulfide isomerase or thiol-protease/Danisco) with masa or corn prior to nixtamalization such that the native protein is modified. Sahai and Jackson (2001) disclosed a similar process where whole-kernel corn was steeped and digested with a food-grade commercial alkaline-protease ( less than 0.10%: 50xc2x0 C.-60xc2x0 C.; pH greater than 10) which altered zein structure similarly to alkali-cooking with a partial gelatinization (xcx9c20%-40%).
A novel transgenic thermostable-reductase enzyme was cloned in corn (high-protein) mainly to enhance extractability and recovery of starch and protein important in flaking grit production and in masa production. Reduction of protein disulphide bonds alters the nature of corn flour (as a wheat substitute from high-protein corn) when steeping the corn grain between 45xc2x0 C. and 95xc2x0 C. instead of using sulfites. The critical steeping is required to soften the kernel and then to loosen starch granules from the complicated matrix of proteins and cell wall material that makes up the corn endosperm (WO. Patent 01/98509).
Tortilla is the main edible corn product in North and Central America. It is a flat, round, unleavened and baked thin pancake (flat-cornbread) made from fresh masa or corn dough prepared from industrial nixtamalized corn flour (masa flour). It might be mentioned that tortilla, when manually or mechanically elaborated and without additives of any kind, has a maximum shelf life of 12 to 15 hours at room temperature. Afterwards they are spoiled by microorganisms and become hard or stale (starch retrogradation) due to a physicochemical change in the starch constituent of either stored or reheated tortilla. It is known that tortillas when kept under conditions in which no moisture is lost (plastic package), nevertheless become inflexible with time and break or crumble easily when bent.
In northern South America, particularly in Colombia and Venezuela, hard endosperm corn is processed with dry milling technology without wastewater and it is further converted into a precooked, degermed and debranned flour for traditional corn foods. Its consumption is mainly in the form of xe2x80x9carepaxe2x80x9d, which is a flat or ovoid-shaped, unleavened, and baked thick pancake made from instant corn flour. In other South American countries, corn meal and corn flour are used for different bakery and pancake mixes as well as snack foods.
Properly processed industrial corn or masa flour simplifies the production of tortilla products, because the customer eliminates management techniques required for wastewater treatment, securing, handling and processing corn into masa for tortillas and snacks. However an instant corn flour might have the following quality and cost disadvantages: high cost, lack of flavor and poor texture in tortilla products or snacks prepared from masa flour.
Corn processors can generate added value from their industrial operations in one of three approaches: developing new products from new hybrids, increasing the yield of traditional products from corn, and improving process efficiency at a lower unit cost. In the past, this has been done by methods and using an apparatus in which the grain is cooked and/or steeped in a lime-water solution such as those disclosed in U.S. Pat. Nos. 2,584,893, 2,704,257, 3,194,664, and 4,513,018. These prior art methods for the industrial production of corn dough or masa flour involve accelerated cooking and steeping times with large amounts of solids losses (xcx9c1.5-2.5%) in the liquid waste. In addition, essential nutrients such as vitamins and some amino acids are lost, depending on the severity of the cooking, washing and drying operations.
Many and varied methods for the production of instant corn flour for food products involving reduced amounts of water with low-temperature cooking and short-time processing for a high yield of the end product have been developed, as reflected by the following U.S. Pat. Nos: 4,594,260, 5,176,931, 5,532,013, and 6,387,437. In this connection, reference is made to the following U.S. Pat. Nos: 4,594,260, 5,176,931, 5,532,013, and 6,265,013 also requiring a low-temperature drying. On the contrary, U.S. Pat. Nos: 4,513,018, 5,447,742 5,558,898, 6,068,873, 6,322,836, and 6,344,228 used a high-temperature dehydration or cooking in place of a low-temperature cooking.
Having in mind the disadvantages of the prior art methods, several studies not only have used a low-temperature precooking with minimum wastewater, but also separate corn fractions as reflected by the following U.S. Pat. Nos: 4,594,260, 5,532,013, 6,025,011, 6,068,873, 6,265,013, and 6,326,045.
A few applications for enzymatic steeping were also tested to convert a traditional masa processing with reduced wastewater into a novel biochemical process (WO Patent 00/45647 and Sahai et al., 2001) Although the above described prior art methods are capable of protease precooking or steeping whole corn for either modified masa or masa flour processing, a continuous industrial application using xylanase instead as a processing aid was still unavailable in the market at the time of the invention.
Accordingly, it is an object of this invention to provide a complete departure from the prior art accelerated precooking methods of thermal, mechanical, chemical and enzymatic or biochemical processing of whole corn in order to control starchy endosperm gelatinization without using chemicals during production of an instant corn flour for arepa and tortilla.
It is another object of this invention to use low-temperature cooking with a microbial xylanase solution for a partial hydrolysis of corn bran heteroxylans during the continuous production of precooked corn flour.
Another object is to use an industrial method and apparatus involving a low-temperature, near neutral-pH precooking which not only solubilize corn cell-walls along with a slower water diffusion effecting a controlled starch granule swelling, but also results in a reduced corn solid loss.
The above and other objects and advantages of the invention are achieved by a new continuous process applied to the production of precooked corn flour or instant corn flour for arepa and tortilla, embodiments of which include a short-time corn precooking followed by a low-temperature and near neutral-pH precooking with a xylanase as a processing aid so as to effect a partial hydrolysis of insoluble fiber and decreased gelatinization, reduced washing and corn solid loss of precooked kernel, stabilization of the moisture content to a desired optimum level for grinding, milling and drying the preconditioned kernel to produce a uniform partial gelatinization, cooling and drying the dry-ground particle, separating and recovering the fine grind so produced from the coarser grind while the latter is further aspirated to partially remove a bran fraction for feed or integral flour, remilling the isolated coarser grind and further sieving it to obtain an instant corn flour for arepa, and admixing only a fine flour with lime to produce masa flour for tortilla and derivatives thereof.